The technical field of this invention is magnetometry and, in particular, the nondestructive electromagnetic interrogation of materials of interest to deduce their physical properties and to measure kinematic properties such as proximity. The disclosed invention applies to both conducting and magnetic media.
Conventional application of magnetometers, specifically eddy current sensors, involves the excitation of a conducting winding, the primary, with an electric current source of prescribed temporal frequency. This produces a time-varying magnetic field at the same frequency. The primary winding is located in close proximity to the material under test (MUT), but not in direct contact with the MUT. This type of nondestructive electromagnetic interrogation is sometimes called near field measurement. The excitation fields and the relevant spatial and temporal variations of those fields are quasistatic. The magnitude and phase (or the real and imaginary parts) of the impedance measured at the terminals of the primary winding (i.e., the measured voltage at the primary winding terminals divided by the imposed current) or the transimpedance (i.e., the voltage measured at a secondary winding terminal divided by the imposed current in the primary winding) is used to estimate the MUT properties of interest.
The time-varying magnetic field produced by the primary winding induces currents in the MUT that produce their own magnetic fields. These induced fields have a magnetic flux in the opposite direction to the fields produced by the primary. The net result is that conducting MUTs tend to exclude the magnetic flux produced by the primary windings. The measured impedance and transimpedance at the terminals of the sensor windings are affected by the following: the proximity to the MUT, the physical properties (e.g., permeability and conductivity) of the MUT and the spatial distribution of those properties; the geometric construct of the MUT; other kinematic properties (e.g., velocity) of the MUT; and the existence of defects (e.g., cracks, corrosion, impurities).
The distribution of the currents induced within conducting MUTs and the associated distribution of the magnetic fields in the MUT, in the vicinity of the MUT, and within the conducting primary and secondary windings are governed by the basic laws of physics. Specifically, Ampere's and Faraday's laws combined with Ohm's law and the relevant boundary and continuity conditions result in a mathematical representation of magnetic diffusion in conducting media and the Laplacian decay of magnetic fields. Magnetic diffusion is a phenomena that relates the distribution of induced currents in conducting materials to the distribution of the imposed and induced magnetic fields. Laplacian decay describes the manner in which a magnetic field decays along a path directed away from the original field source.
Magnetometers, such as eddy current sensors, exploit the sensitivity of the impedance or transimpedance (measured at the sensor winding terminals) to the physical and geometric properties of the MUT. This is sometimes accomplished by using multiple temporal excitation frequencies. As the primary winding excitation frequency is increased, the currents in a conducting MUT exclude more and more flux until all the induced currents in the MUT are confined to a thin layer near the surface of the MUT. At frequencies for which the induced currents are all at the surface of the MUT, the MUT can be represented theoretically as a perfect conductor. In other words, at high enough frequency, variations in the conductivity of the MUT will no longer affect the impedance or transimpedance measured at the sensor windings.
This effect has been used in proximity measurement relative to a conducting media. Measurement of proximity to a metal surface is possible at a single excitation frequency, if that frequency is high enough that the MUT can be treated as a perfect conductor. For proximity measurement at lower frequencies, it is necessary to account for the effects of the conductivity of the MUT on the measured impedance, either by physical modeling or by calibration.
In applications requiring the measurement of conductivity, it is necessary to operate at frequencies low enough that the measurements at the terminals of the conducting windings are sensitive to the MUT conductivity. Such applications include the monitoring of aging in conducting media, as well as the direct measurement of conductivity for quality monitoring in metal processing and manufacturing process control. For example, the accurate measurement of the case depth (e.g., the thickness of a heat-affected zone at the surface of a metal after heat treatment) requires a sensor winding geometry and excitation conditions (e.g., frequency, proximity to the MUT) that produce the required sensitivity to the conductivity and thickness of the heat-affected zone.
Two methods are available for determining the desired conditions: (1) experimentation and calibration, and (2) physical modeling and response prediction from basic principals. In practice, each of these techniques has met with some success. The principal limitations of experimentation and calibration are the need for fabrication of expensive calibration test pieces (standards) for each new application, the relatively small dynamic range (i.e., the small range of permissible MUT property variations over which the measurement specifications can be met), and the inaccuracies produced by variation in uncontrolled conditions such as temperature and lift-off errors.
The principal limitations of the physical modeling approach are the inaccuracies introduced by modeling approximations and the existence of unmodeled effects. These limitations are most severe for sensor winding constructs that are not specifically designed to minimize unmodeled effects.
In spite of these limitations, the successful use of conducting windings driven by a current source, as in eddy current sensors, to measure physical and kinematic properties has been widely demonstrated.
For example, eddy current sensors have been used to measure the thickness of conducting strips of known conductivity, as disclosed in Soviet Patents 578,609 and 502,205. Eddy current sensors have also been used for flaw detection, as disclosed in U.S. Pat. No. 3,939,404. Other eddy current sensor applications include measurement of the conductivity-thickness product for thin conducting layers, measurement of the conductivity of conducting plates using calibration standards, and measurement of proximity to conducting layers. Such sensors are also used in proximity measurement for control of machines and devices.
The ability to resolve distributions of parameters and properties of different layers in multi-layered materials has been addressed in U.S. Pat. No. 5,015,95 1. The, referenced patent introduced the concept of multiple wave-number magnetic interrogations of the material of interest, by imposing several different spatial magnetic field excitations, using multiple preselected sensor winding constructs, each with a different wavelength.